describes the presence of different "sites" of phosphorylation on a particular protein, that are catalyzed by different kinsases/phosphatases

explains complicated regulation of protein function

amplification potential of protein kinase

1. Each kinase (or phosphatase) can catalyze the phosphorylation (dephosphorylation) of many substrate polypeptide molecules

if the phosphorylated (dephophorylated) substrate is an enzyme that is activated by such modification...

each enzyme in turn can generate many substrate products

2. ACTIVATED ENZYME(s) MAY HAVE KINASE/PHOSPHATASE ACTIVITY Many protein kinases/phosphatases are subject to covalent modification by phosphorylation: this created "cascades" of kinase and phosphatase activation/inactivation which also contributes to further amplification